1. Field of the Invention
The present invention relates to a device for controlling brakes of a vehicle such as an automobile, and more specifically, to such a device that controls braking force distribution among front and rear wheels in a vehicle.
2. Description of Prior Art
During braking of a vehicle, a rear wheel is liable to be locked because its frictional circle is shrunk due to the forward shifting of the load of the vehicle. The locking of a rear wheel, prior to a front wheel, induces serious deterioration of a vehicle running behavior, such as disturbance in the attitude and/or spinning of a vehicle body. In order to avoid the locking of a rear wheel, braking force distribution (BFD) control has been proposed to keep braking force on rear wheels lower than on the front wheels. In such BFD control, braking pressures applied to rear wheel cylinders in a hydraulic braking system is held, reduced or pulsatively increased, i.e. the increase in braking force generated on rear wheels is restricted, providing a distribution of braking force biased to front wheels. Usually, BFD control, often referred to as “Electronic Braking force Distribution Control (EBD control)”, is executed by a computerized device operating a plurality of solenoid valves in a hydraulic circuit. Examples of devices executing EBD control are seen in Japanese Laid-Open Patent Publications (JP) Nos. 5-213169 and 2001-219834.
Under EBD control, a demand from a driver of a vehicle for increasing braking force (e.g. a depression of a brake pedal) is modified for the restriction of braking force on rear wheels, which would cause the reduction of the total braking force. Thus, the driver would feel that the actually generated braking force is incompatible with his braking operation. In order to eliminate this feeling of the incompatibleness while maintaining the braking performance and behavior of a vehicle, JP No. 2001-219834 discloses an EBD control device, in which, after once restricted, braking force on rear wheels is pulsatively increased in response to the increase of a braking action by a driver under a certain running condition. For preventing the locking of rear wheels, however, braking force on the rear wheels could not be increased limitlessly.
Accordingly, it is preferable that, in BFD control device, total braking force on a vehicle body may be rendered as close to the amount requested by a driver of the vehicle as possible, without inducing the locking of the rear wheels and the instability in the vehicle attitude induced therefrom.
By the way, even during execution of BFD control, Anti-skid (ABS) control is to be executed if either of wheels is being locked (Because of BFD biased to front wheels, usually, a front wheel is more liable to be locked than a rear wheel.). During ABS control, decreasing and increasing braking pressure for a wheel to be controlled is repeated to adjust the slip ratio of the wheel, and thereby the slipping or locking of the wheel is prevented. Since a condition requiring ABS control is to be controlled urgently, ABS control will operate a hydraulic circuit of a brake system to change wheel cylinder pressures while ignoring the setting of the pressure by BFD control. Further, when the front wheel is under ABS control, the rear wheel braking pressure is often increased for compensating for the restriction of the front wheel braking force and obtaining total braking force on a vehicle. In such a case, namely, when BFD and ABS controls are executed subsequently and/or alternatively, the condition in the hydraulic circuit is liable to be out of control of a driver, causing deterioration of the braking controllability of the vehicle.
Accordingly, a BFD control device should be improved in order to render it compatible with ABS control without deteriorating the controllability of the vehicle as well as the advantages of both BFD and ABS controls.